The present invention relates generally to a process for calcining green coke obtained by a delayed coking process, and more specifically to a process for producing, with high thermal efficiency, high-grade coke suitable for use in the preparation of graphite electrodes.
Preparation of green coke from heavy oils of petroleum origin such as residual oils of catalytic cracking and thermal cracking, straight run residual oils and tar of thermal cracking, coal tar pitch or mixtures thereof, by a delayed coking process is known in the art. The green coke produced by this process still contains a significant quantity of moisture and volatile matter. Accordingly, there is also known a process for calcining the produced green coke in order to remove the moisture and volatile matter from the green coke and to densify the same, thereby producing a carbon material having a high density and a low coefficient of thermal expansion which is suitable for use as an electrode material for steel-making, aluminum smelting or the like, or a carbon material for other shaped articles.
Calcining of such green coke is carried out in heating furnaces such as a rotary kiln, a rotary hearth, and a shaft kiln in a single stage, or in two stages by further providing a preheating furnace.
We have found, as a result of our research on the unit stages in the calcination of coke, that one or two stages of heating furnaces are insufficient, and that at least three stages of heating furnaces are necessary in order to produce high-grade coke efficiently, and have developed a process for calcining coke (Japanese Patent Laid-Open Publication No. 10301/1979, U.S. patent application Ser. No. 890,707). More particularly, this calcining process comprises calcining green coke obtained by a delayed coking process in heating furnaces of at least three stages connected in series, in which the control of temperature and the adjustment of the atmosphere in the respective furnaces can be independently carried out, which process comprises carrying out the following steps in the respective furnaces in the indicated order:
(a) evaporating the water contained in the green coke, and drying and preheating the coke;
(b) distilling off and burning the volatile matter in the dried coke; and
(c) heating and calcining the coke from the step (b).
As far as we are aware, the calcined coke obtained by this process is not fully satisfactory as coke for artificial graphite electrodes which needs to be of particularly high grade. That is, there remains much room for improvement with respect to high density and low coefficient of thermal expansion, which are the most important properties required of coke for artificial graphite electrodes.
On the other hand, it has been found that cooling in an intermediate stage in the calcination of coke is highly effective in reducing the coefficient of thermal expansion of the calcined coke and increasing the density, particularly the true density thereof, and a process has been developed for producing high-grade coke. This process for calcining coke comprises first calcining green coke obtained by a delayed coking process at a temperature lower than an ordinary calcining temperature, cooling once the calcined coke, and thereafter calcining the coke again at a temperature in the ordinary calcining temperature range (as disclosed in U.S. Pat. No. 4,100,265, July 11, 1978). Although it is not sufficiently clear why the coefficient of thermal expansion of the calcined coke is reduced by intermediate cooling, a possible reason may be that some fine cracks are formed in the coke during the process wherein the coke, after being heated to a temperature of 600.degree. to 1000.degree. C., is subjected to intermediate cooling and then to reheating, which cracks are considered to absorb expansion due to heating, resulting in the reduction of the overall coefficient of thermal expansion of the coke. The true density of the calcined coke is increased presumably because rapid evaporation of volatile matter and formation of a porous structure which occurs as a result thereof are suppressed by the intermediate cooling in the above specified temperature range.
It may appear that calcined coke of higher grade can be obtained by applying this concept to the process disclosed in the aforementioned U.S. patent application Ser. No. 890,707 (hereinafter referred to as "original three-stage process"), i.e., by once cooling the preliminarily calcined coke from the step (b) in the original three-stage process, and then calcining the coke in the step (c). However, this is not as easy as might be expected because, in order to once cool the preliminarily calcined coke from the step (b), then to reheat the coke to a temperature equal to the temperature of the outlet of the furnace for the step (b), and then further to supply heat required for the final calcination, the heating furnace for the step (c) will be too heavily loaded, and the quantity of the sensible heat of the waste gas obtained by the increased load will be so great that it cannot be consumed in the entire calcination system. Thus, the application of the original three-stage process to the two-stage calcining process according to the aforementioned U.S. Pat. No. 4,100,265, which comprises intermediate cooling, has been considered unpractical.
As a result of our extended research, however, it has been found that, by suppressing to a minimum the combustion of the volatile matter evaporated in the step (b) in the original three-stage process described above, and, instead of using the waste gas from the step (b) as a gas for drying and preheating the coke in the step (a) as in the original three-stage process, introducing this waste gas into the step (c) where the gas is burned and utilized as a heat source for the final calcination of the coke, the overall sensible heat of the waste gas is not greatly increased, even if the heat load in the step (c) is increased, and thus can be utilized in the system.
It has further been found that, by suppressing the combustion of the volatile matter in the step (b), it becomes easier to control the temperature at the outlet through which the calcined coke from the step (b) is discharged. It should be noted that this control is the most important problem encountered in the two-stage calcining process. In order to suppress the combustion of the volatile matter in the step (b), it is sufficient to introduce air only in the minimum quantity required for the combustion of fuel to generate heat necessary for the preliminary calcination in the step (b), and to maintain the system under a non-oxidizing atmosphere.